1. Field of the Invention
The present invention relates to an antireflection coating film formed on an optical base member such as a lens, for example, and an optical element, an optical system, an optical apparatus having the same.
2. Description of the Related Art
In the related art, antireflection measures are taken for reducing light quantity loss of an incident light on a surface of a lens. For example, as one of the antireflection measures for optical elements for visible light, a dielectric multilayer film referred generally to as a multi-coat is widely used. The dielectric multilayer film is configured to reduce reflected light by laminating thin films having different refractive indexes and having adequate film thickness, adjusting amplitudes and phases of reflected waves generated at surfaces and interfaces of the respective films and causing the reflected waves to interfere with each other. Therefore, superior antireflection performances with respect to light rays having specific wavelengths and incident angles are achieved. However, as regards other light rays, interference conditions are not fulfilled, it is difficult to achieve high antireflection performances over a wide wavelength band range or a wide incident angle range.
In contrast, in digital cameras being in widespread use in recent years, image sensors such as CCD or CMOS having higher reflectance in comparison with silver-halide film of the related art are used. Therefore, specific undesirable light, referred to as “digital ghost” caused by light reflected from a sensor surface reflects again on the lens surface, and reaches the sensor surface again may often occur.
In addition, as the lenses for the digital cameras, anomalous dispersion glasses, aspherical lenses, the lenses having large curvatures tend to be used in order to achieve both high image quality and high specifications (such as zoom magnification or brightness) and portability (being compact and light weight). Among these lenses, the lenses having the large curvatures cause light rays incident at large angles in a periphery thereof, and hence the dielectric multilayer film of the related art is not enough to reduce the reflection. Consequently, the undesirable light which deteriorates a quality of shot images such as flare and ghost may occur.
In view of such circumstances, development of a high performance antireflection coating film superior in wavelength band range characteristics, and incident angle characteristics in comparison with the dielectric multilayer film is required. U.S. Patent Application Publication No. 2005/0225878 discloses an antireflection coating film formed with a magnesium fluoride layer by a sol-gel method on a dielectric thin film including three layers formed by using a vacuum vapor deposition method, and that superior antireflection characteristics are achieved by adequately setting the refractive indexes and film thicknesses of the layers from a first layer to a fourth layer.
However, in the antireflection coating film disclosed in above-described U.S. Patent Application Publication No. 2005/0225878, the reflectance of the light ray having a wavelength of 550 nm and the indicant angle of 0° has a value on the order of 0.4%, which cannot be said to bring out sufficient antireflection performances. Also, the reflectance of the light ray having a wavelength of 550 nm and an indicant angle of 60° has a value on the order of 2%, which cannot be said to bring out sufficient incident angle characteristics.